The crosslinking of vinyl containing polymers with sulfur in the presence of various catalysts, accelerators and additives is well known. The sulfur induced crosslinking of the vinyl containing polymers generally involves the formation of mono-, di-, and tri-sulfido bridges between vinyl groups on the polymer backbone, resulting in the vulcanization of the polymer. The sulfide crosslinking imparts greater strength and resiliency to the polymer, allowing for its subsequent use in a variety of demanding applications. The method requires the continuous heating of the polymer and is therefore energy intensive.
The crosslinking of vinyl containing polymers with organic peroxides is also well known. The mechanism of organic peroxide induce crosslinking involves the generation of radicals that abstract hydrogen atoms from the polymer, thereby further generating polymer-bound radicals which link together to form covalent carbon-carbon bonds. The formation of the carbon-carbon bonds produces the crosslinking of the polymer necessary to impart greater strength and resiliency to the vulcanized substrate. Organic peroxide crosslinking typically is used when heat age resistance or continuous vulcanization is desired. This process also requires the continuous input of heat energy.
A third method of crosslinking vinyl-containing polymers involves the use of moisture crosslinkable silanes, such as vinylalkoxysilanes, which have been grafted onto the polymer backbone via a peroxide coupling reaction. The process involves a peroxide induced grafting reaction of a vinylalkoxysilane onto the polymer backbone and the blending of the alkoxysilane hydrolysis catalyst, such as a tin compound. The graft polymer is then submitted for the fabrication step. The crosslinking of the vinyl containing polymer is initiated when the fabricated article is exposed to a source of moisture. The rate of crosslinking is, however, dependent upon mass transfer of moisture into an inherently hydrophobic polymer matrix. This method of crosslinking is subsequently limited to fabricated articles having high surface area/volume ratios, such as is found in thin walled pipe or certain cable insulations.
An alternative to the aforementioned crosslinking methods is taught in GB No. 1,118,327 which involves the use of multi-SiH containing siloxanes and a platinum hydrosilation catalyst to promote the hydrosilation crosslinking of vinyl containing polymers. The vinyl containing polymers described in GB No. 1,118,327 are primarily ethylene/propylene/diene monomer terpolymers (EPDM). The source of pendant unsaturation in EPDM rubbers are di-unsaturated, unconjugated olefins such as ethylidene norbornene, dicyclopentadiene, 1,5-cyclooctadiene, 1,4-hexadiene and 1,7-octadiene. The hydrosilation crosslinking of the vinyl-containing polymer is achieved via the addition of multiple SiH groups of an individual siloxane molecule to the pendant vinyl groups of the EPDM terpolymer. The network of Si-C bonds formed from the hydrosilation reaction ultimately results in the crosslinking of the terpolymer. GB No. 1,118,327 describes the use of a specific class of organohydridopolysiloxanes of the general formula: EQU [R.sub.m SiO.sub.(4-m)/2 ]
where at least five units of the molecule are HR.sub.n SiO.sub.3-n/2 where n=1 or 2, m=0, 1, 2 or 3 and R is a monovalent hydrocarbon radical free from aliphatic unsaturation. The most useful structure is said to be the siloxane having the general formula: EQU (CH.sub.3).sub.3 SiO--(CH.sub.3 HSiO).sub.x --Si(CH.sub.3).sub.3
where x has an average value of 10 to 90. Catalysts disclosed as useful in the process are various forms of platinum hydrosilation catalysts, such as olefin complexed platinum, platinum complexed with sym-1,2-divinyl-1,1,2,2-tetramethyldisiloxane or chloroplatinic acid. This method is found to result in bubble formation within the fabricated articles, thereby making its use unattractive.
The use of Group VIII transition metal complexes as hydrosilation catalysts has been well documented in the literature as described in Organometallic Chemistry Reviews No. 5, by Lukevics et al. pp 1-179 (1977). For the most part, H.sub.2 PtCl.sub.6.6H.sub.2 O dissolved in an alcohol such as ethanol or isopropanol, is the most widely used hydrosilation catalyst. The use of other platinum complexes with a wide variety of attached ligands, such as organophosphines, organosulfides, unsaturated organics such as alkenes have also been extensively used. Other Group VIII transition metal complexes containing similar ligands have also been described as catalysts for the hydrosilation reaction. These catalysts and their use in the hydrosilation reaction are well known by those skilled in the art. However, some of these hydrosilation catalysts will also catalyze various side reactions, many of which lead to the production of volatile by-products. These by-products can then cause undesirable bubble formation within the polymeric substrate.
In addition to Group VIII transition metal catalysts, a number of inhibitors for the hydrosilation reaction are also known. The use of the inhibitors for the hydrosilation reaction stems from the high activity of the Group VIII transition metal catalysts; so active that some compositions containing the vinyl containing substrate, the SiH-containing substrate and the hydrosilation catalyst undergo the hydrosilation reaction even at ambient temperatures. The incorporation of a hydrosilation catalyst inhibitor thereby improves the shelf life stability of the composition at ambient temperatures.
The hydrosilation crosslinking of vinyl containing polymers is not limited to the multi-SiH containing siloxanes or the EPDM terpolymers as described in GB No. 1,118,327. Hydrosilation crosslinking has also been reported for polymers such as polyisobutylene functionalized with terminal unsaturation utilizing various multi-SiH containing siloxanes and platinum hydrosilation catalysts. In particular, HMe.sub.2 SiOMe.sub.2 SiOSiMe.sub.2 H and Si(OSiMe.sub.2 H).sub.4 (described in Polymer Bulletin 1 575 (1979)); [MeHSiO].sub.5 (described in Macromolecules 13 681-685 (1980)); and HMe.sub.2 Si(Me.sub.2 SiO).sub.n OSiMe.sub.2 H where n=3-7 (described in the Abstracts of the 20th Organosilicon Symposium, Tarrytown, N.Y. (1986)) are all examples of other multi-SiH containing siloxanes useful in the crosslinking of olefins.
Most recently, a hydrosilation crosslinked vinyl-containing polymer composition was described in JP No. 61.60,727. Specifically, it discloses the crosslinking of a polyolefin containing terminal or pendant unsaturation via a hydrosilation reaction using siloxanes containing preferably greater than 10 organohydrogensiloxane units per siloxane molecule and a platinum catalyst. It further discloses a method to avoid the generation of hydrogen gas and therefore bubble formation due to a deleterious side reaction between the multi-SiH containing siloxane and the platinum hydrosilation catalyst.